Functional fluid compositions containing erosion inhibitors

ABSTRACT

A functional fluid composition containing a fluid base stock (e.g., a phosphate ester fluid base stock) as a major component, and an erosion inhibitor mixture as a minor component. The erosion inhibitor mixture contains at least a first erosion inhibitor (e.g., a perhalometallate or perhalometalloidate salt) and a second erosion inhibitor (e.g., an alkali metal salt of perfluoroalkyl sulfonic acid). Erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304. A method for operating and lubricating a hydraulic system by utilizing as a hydraulic fluid, the functional fluid composition. A method for identifying erosion inhibition effectiveness of the functional fluid. The functional fluids are useful as aircraft hydraulic fluids.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 62/264,092 filed Dec. 7, 2015, which is herein incorporated by reference in its entirety.

FIELD

This disclosure relates to a functional fluid composition containing a fluid base stock (e.g., a phosphate ester fluid base stock) as a major component, and an erosion inhibitor mixture as a minor component. The erosion inhibitor mixture contains at least a first erosion inhibitor (e.g., a perhalometallate or perhalometalloidate salt) and a second erosion inhibitor (e.g., an alkali metal salt of perfluoroalkyl sulfonic acid). This disclosure also relates to a method for operating and lubricating a hydraulic system by utilizing as a hydraulic fluid, the functional fluid composition. This disclosure further relates to a method for identifying erosion inhibition effectiveness of the functional fluid. The functional fluids are useful as aircraft hydraulic fluids.

BACKGROUND

Functional fluids have been utilized as electronic coolants, diffusion pump fluids, lubricants, damping fluids, bases for greases, power transmission and hydraulic fluids, heat transfer fluids, heat pump fluids, refrigeration equipment fluids, and as a filter medium for air-conditioning systems. Phosphate ester-based functional fluids have been recognized for some time as advantageous for use as the power transmission medium in hydraulic systems. Such systems include recoil mechanisms, fluid-drive power transmissions, and aircraft hydraulic systems.

Hydraulic fluids intended for use in the hydraulic system of aircraft for operating various mechanisms and aircraft control systems must meet stringent functional and use requirements. Phosphate ester-based fluids find particular utility in aircraft hydraulic fluids because of their special properties which include high viscosity index, low pour point, high lubricity, low toxicity, low density and low flammability. Thus, for many years, numerous types of aircraft, particularly commercial jet aircraft, have used phosphate ester-based fluids in their hydraulic systems. Among the most important requirements of an aircraft hydraulic fluid is that it be stable against oxidative and hydrolytic degradation at elevated temperatures.

In addition, functional fluids for use in aircraft hydraulic systems must be capable of performing in the hydraulic system over an extended period of time without causing significant damage or functional impairment to the various conduits, valves, pumps, and the like, through which the functional fluid flows in the course of such use. Damage caused by functional fluids contacting valves and other members has been attributed to streaming current induced corrosion or erosion, by the environment in contact with the functional fluid in a hydraulic system.

The hydraulic systems of a typical modern aircraft contain a fluid reservoir, fluid lines and numerous hydraulic valves which actuate various moving parts of the aircraft such as the wing flaps, ailerons, rudder and landing gear. In order to function as precise control mechanisms, these valves often contain passages or orifices having clearances on the order of a few thousandths of an inch or less through which the hydraulic fluid must pass. In a number of instances, valve orifices have been found to be substantially eroded by the flow of hydraulic fluid. Erosion increases the size of the passage and reduces below tolerable limits the ability of the valve to serve as a precision control device.

For example, aircraft have experienced slow response of flight controls as a result of valve erosion. Thus, phosphate ester-based aircraft hydraulic fluids require use of an erosion inhibitor, i.e. a functional fluid additive which prevents or inhibits the erosion of hydraulic system valves. Other additives which perform special functions such as hydrolysis inhibition, viscosity index improvement and foam inhibition are also frequently present in such hydraulic fluid.

Erosion inhibition additives are used at very small concentration in the aircraft hydraulic fluids. The primary function of anti-erosion additive is to provide proper electrical conductivity to the system to reduce the wall current of servo valves. Electro-chemical erosion is a phenomenon peculiar to esters, especially phosphate esters. It occurs due to the highly polar nature of the fluid. This phenomenon is actually corrosion of ferrous metal at locations where the fluid flows at high velocity. Air craft builders design specific erosion test to ensure adequate anti-erosion property of the hydraulic fluids. For example, Boeing erosion test is a 500 hour test at 225° F.-275° F. range with the injection of certain amount of chlorine (a contaminant which can accelerate the erosion mechanism, typically used at 200 to 1000 ppm) to measure the internal leakage rate in a test servo valve. Airbus erosion test uses a condition of 200 ppm chlorine injection at 205° F. and test for 1000 hours.

Before fluid providers engage in these expensive erosion tests, a simple conductivity test could serve the purpose as effective screening tool. It is strongly believed that all anti-erosion additives possess good conductivity. The conductivity is measured by conductivity meter where the electrical conductivity (K) is the reciprocal of resistance. It is measured on one cubic centimeter of fluid at a specified temperature. The units are micromhos/cm or microSiemens/cm. According to a published SAE presentation by Nelson, Waterman in 1974 entitled “Advancement in Commercial Airplane Hydraulic Fluids” (SAE committee A-6) and other publications by Beck and Olsen (Wear of Small Orifices by Streaming Current Driven Corrosion—Boeing Scientific Research Laboratories, ASME, J. Basic Engineering, 92, 4, pp 782-791, 1970; Pitting and Deposits with an Organic Fluid by Electrolysis and by Fluid Flow, J. Electrochemical Soc., 119, 2, pp 155-160, 1972), it was known that high conductivity fluid can minimize the erosion issue. The electrical properties of the phosphate ester hydraulic fluids determine the rate of erosion and if the electrical conductivity is greater than approximately 0.3 micro-mhos/cm, no or very little erosion will occur.

Due to the highly ionic nature, many of the anti-erosion additives are extremely sensitive to trace amount of water present in the system. Although hydrolysis inhibitors (water scavengers) are typically used in the aircraft hydraulic fluids to ensure the water ingress can be fully under control, the intrinsic resistance of water by an erosion inhibition additive system is an important characteristic when it comes to select the proper anti-erosion additive. Since phosphate esters are mildly hygroscopic, the hydraulic fluid containers should remain closed when not in use to keep moisture in the air from absorbing into the fluids. Karl Fisher coulometric titration method (ASTM D6304) is widely used in the industry and effective in quantifying the trace amount of water present in the fluids.

Acidity is also an important test that defines fluid life. Acid scavenging agent is typically used to control acidity, which usually keeps the level under 0.5 mg KOH/g. When acid reaches the range higher than 1.0 mg KOH/g, it means the system control is severely depleted. ASTM D974 is a titration method widely used to measure the acidity of the fluid. The method uses a potentiometric titrimeter for standardized base (KOH) to neutralize the acidity of oil sample to determine acid constituents in new and used lubricants. As a result of hydrolysis, many erosion inhibition additives could liberate corresponding acid from the salt form; therefore, an increase of acidity of the fluid may occur.

In the presence of sulfur containing additives, such as perfluorooalkyl sulfonate (PFAS) and perfluorooctyl sulfonate (PFOS), copper corrosion is a concern; therefore, a typical ASTM copper corrosion test will be used to check the corrosion resistance of the fluid.

Current commercial phosphate ester-based aircraft hydraulic fluids utilize a single erosion inhibitor at a very narrow concentration range. It would be desirable to have alternative erosion inhibitors available for use in phosphate ester-based aircraft hydraulic fluids, especially improved erosion inhibitors with good copper corrosion resistance, acidity control, electrical conductivity, and hydrolytic stability. Furthermore, a well-balanced overall performance feature built on the four tests mentioned above is desirable to the selection of proper erosion inhibition system.

SUMMARY

This disclosure relates to a functional fluid composition containing a fluid base stock (e.g., one or more phosphate ester fluid base stocks) as a major component, and an erosion inhibitor mixture as a minor component. The erosion inhibitor mixture contains at least a first erosion inhibitor (e.g., a perhalometallate or perhalometalloidate salt) and a second erosion inhibitor (e.g., an alkali metal salt of perfluoroalkyl sulfonic acid). This disclosure also relates to a method for operating and lubricating a hydraulic system by utilizing as a hydraulic fluid, the functional fluid composition. This disclosure further relates to a method for identifying erosion inhibition effectiveness of the functional fluid. The functional fluids are useful as aircraft hydraulic fluids.

This disclosure also relates in part to a functional fluid composition comprising:

(a) a fluid base stock as a major component, wherein the fluid base stock comprises one or more phosphate ester fluid base stocks represented by the formula

wherein R¹, R² and R³ are independently a substituted or unsubstituted alkyl or aryl group; and

(b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises at least a first erosion inhibitor and a second erosion inhibitor; wherein the first erosion inhibitor comprises a perhalometallate or perhalometalloidate salt represented by the formula:

M[AX_(y)]_(z)

wherein A is a metal or metalloid; M is a solubilizing cation; X is a halogen; y is an integer from 1 to 7 and equal to the positive valence of A; and z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral; and wherein the second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid represented by the formula

R₁SO₃M

where M is an alkali metal, and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18. Erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.

This disclosure further relates in part to a method for operating and lubricating a hydraulic system by utilizing as a hydraulic fluid a functional fluid comprising:

(a) a fluid base stock as a major component, wherein the fluid base stock comprises one or more phosphate ester fluid base stocks represented by the formula

wherein R¹, R² and R³ are independently a substituted or unsubstituted alkyl or aryl group; and

(b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises at least a first erosion inhibitor and a second erosion inhibitor; wherein the first erosion inhibitor comprises a perhalometallate or perhalometalloidate salt represented by the formula:

M[AX_(y)]_(z)

wherein A is a metal or metalloid; M is a solubilizing cation; X is a halogen; y is an integer from 1 to 7 and equal to the positive valence of A; and z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral; and wherein the second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid represented by the formula

R₁SO₃M

where M is an alkali metal, and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18. Erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.

This disclosure yet further relates in part to a method for identifying erosion inhibition effectiveness of a functional fluid. The method comprises:

providing a functional fluid comprising:

(a) a fluid base stock as a major component, wherein the fluid base stock comprises one or more phosphate ester fluid base stocks represented by the formula

wherein R¹, R² and R³ are independently a substituted or unsubstituted alkyl or aryl group; and

(b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises at least a first erosion inhibitor and a second erosion inhibitor; wherein the first erosion inhibitor comprises a perhalometallate or perhalometalloidate salt represented by the formula:

M[AX_(y)]_(z)

wherein A is a metal or metalloid; M is a solubilizing cation; X is a halogen; y is an integer from 1 to 7 and equal to the positive valence of A; and z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral; and wherein the second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid represented by the formula

R₁SO₃M

where M is an alkali metal, and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18;

determining electrical conductivity of the functional fluid in accordance with ASTM D2624;

determining corrosiveness to copper of the functional fluid in accordance with ASTM D130;

determining total acid number of the functional fluid in accordance with ASTM D974;

determining entrained water content in the functional fluid in accordance with ASTM D6304; and

identifying erosion inhibition effectiveness of the functional fluid based on results from one or more of ASTM D2624, ASTM D130, ASTM D974 and ASTM D6304.

This disclosure also relates in part to a functional fluid composition comprising:

(a) a tri-n-butyl phosphate fluid base stock as a major component, and

(b) an erosion inhibitor mixture as a minor component, in which the erosion inhibitor mixture comprises potassium hexafluorophosphate and potassium trifluoromethane sulfonate. Erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the potassium hexafluorophosphate or the potassium trifluoromethane sulfonate, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.

It has been surprisingly found that, in accordance with this disclosure, a phosphate ester functional fluid composition having a particular erosion inhibitor mixture (i.e., at least a first erosion inhibitor comprising a perhalometallate or perhalometalloidate salt and a second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid) exhibits improved erosion inhibition as compared to erosion inhibition exhibited using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.

Other objects and advantages of the present disclosure will become apparent from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows fluid compositions and testing results of the fluid compositions in accordance with Example 1.

FIG. 2 shows fluid compositions and testing results of the fluid compositions in accordance with Example 2.

FIG. 3 shows fluid compositions and testing results of the fluid compositions in accordance with Example 3.

FIG. 4 shows fluid compositions and testing results of the fluid compositions in accordance with Example 4.

FIG. 5 shows fluid compositions and testing results of the fluid compositions in accordance with Example 5.

FIG. 6 shows fluid compositions and testing results of the fluid compositions in accordance with Example 6.

FIG. 7 shows fluid compositions and testing results of the fluid compositions in accordance with Example 7.

FIG. 8 shows various classifications of copper corrosion tests in accordance with ASTM D130.

DETAILED DESCRIPTION OF THE EMBODIMENTS

All numerical values within the detailed description and the claims herein are modified by “about” or “approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

ASTM D2624 is a standard test method for electrical conductivity of aviation and distillate fuels. ASTM D974 is a standard test method for acid and base number by color-indicator titration. ASTM D130 is a standard test method for corrosiveness to copper from petroleum products by copper strip test. ASTM D6304 is a standard test method for determination of water in petroleum products, lubricating oils, and additives by coulometric Karl Fischer titration. The methodology of using the combination of these four ASTM tests enables a determination of the effectiveness of erosion inhibition systems. Additional tests, such as elemental analysis of metal salts, may further strengthen the overall assessment capability. However, the above four ASTM tests are the foundation of the determination factors. As used herein, the ASTM standard test methods include, in addition to the particular ASTM test method, any equivalent test methods for determining the same or similar properties.

The anti-erosion functional fluids of this disclosure comprise a base fluid, an erosion inhibitor mixture, and optionally other additives. In particular, the functional fluid compositions of this disclosure comprise a fluid base stock (e.g., phosphate ester) as a major component, and an erosion inhibitor mixture as a minor component. The erosion inhibitor mixture comprises at least a first erosion inhibitor (e.g., a perhalometallate or perhalometalloidate salt) and a second erosion inhibitor (e.g., an alkali metal salt of perfluoroalkyl sulfonic acid). Erosion inhibition of the functional fluid composition is surprisingly and significantly improved as compared to erosion inhibition achieved using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.

With regard to electrical conductivity determined according to ASTM D2624, the functional fluids of this disclosure have an electrical conductivity range of greater than about 0.3 to less than about 2.0 micromhos/cm, preferably greater than about 0.4 to less than about 1.7 micromhos/com, more preferably greater than about 0.5 to less than about 1.5 micromhos/com, and most preferably greater than about 0.6 to less than about 1.3 micromhos/cm.

With regard to acidity determined according to ASTM D974, the functional fluids of this disclosure have a total acid number range from about zero to less than about 0.05 mg KOH/g, preferably less than about 0.04 mgKOH/g, more preferably less than about 0.03 mgKOH/g, and most preferably less than about 0.02 mgKOH/g.

With regard to corrosiveness to copper determined according to ASTM D130, the functional fluids of this disclosure have a corrosiveness to copper range of 1A, 1B and 2A, more preferably 1A and 1B, and most preferably 1A.

With regard to determining entrained water content according to ASTM D6304, the functional fluids of this disclosure have a water content range from about zero to less than about 8000 ppm water, preferably less than about 6000 ppm water, more preferably less than about 5000 ppm water, and most preferably less than about 4000 ppm water.

Base Stocks

The functional fluids of this disclosure comprise a fluid base present in major proportion in which the erosion inhibitor mixture and other additives are employed. The fluid base can include a wide variety of base materials, such as organic esters of phosphorus acids, mineral oils, synthetic hydrocarbon oils, silicate esters, silicones, carboxylic acid esters, aromatics and aromatic halides, esters of polyhydric material, aromatic esters (such as polyphenyl ether), thioethers, and the like.

Phosphate esters are preferred base fluids useful this disclosure and have the formula

wherein R¹, R² and R³ independently represent a substituted or unsubstituted alkyl or aryl hydrocarbon group. As used herein, “aryl” includes aryl, alkaryl, and aralkyl structures and “alkyl” includes aliphatic and alicyclic structures. All three groups may be the same, or all three different, or two groups may be alike and the third different. A typical fluid will contain at least one species of phosphate ester and usually will be a mixture of two or more species of phosphate esters.

In an embodiment, the hydraulic fluid base useful in this disclosure comprises a mixture of trialkyl and triaryl phosphate esters with the trialkyl phosphate esters predominating. The trialkyl phosphate esters may be present in amounts of from 70 to 98% by weight of the phosphate ester portion of the total fluid composition. Preferably, the trialkyl phosphate esters will comprise 80 to 92% by weight of the phosphate ester portion of the composition. The trialkyl phosphate esters which give preferred results are those wherein each of the alkyl groups has 1 to 12 carbon atoms and, more preferably, have from 4 to 9 carbon atoms. The alkyl groups may each be either a straight-chain or a branched-chain configuration. A single trialkyl phosphate ester may have the same alkyl group in all three positions, or may have two or three different alkyl groups. Mixtures of various trialkyl phosphate esters may also be used. Suitable, but non-limiting species of trialkyl phosphate esters useful in this disclosure include the tributyl phosphates, particularly tri(n-butyl) phosphate, trihexyl phosphates, trioctyl phosphates, and the like. Particularly preferred are tri(n-butyl) phosphate or the branched-chain isomers of the trioctyl phosphates, such as tri(2-ethylhexyl) phosphate.

The triaryl phosphate esters useful in the compositions of this disclosure may be present in amounts of from about 2 to about 30% by weight of the phosphate ester portion of the total fluid composition. The triaryl phosphate esters which give preferred results are those wherein each of the aryl hydrocarbon groups has between 6 and 15 carbon atoms and, more preferably, from 6 to 10 carbon atoms. These include phenyl groups and alkyl-substituted phenyl groups. As with the trialkyl phosphates, a single triaryl phosphate may have the same aryl groups in all three positions, or may have a mixture of two or three different aryl groups. Various mixtures of triaryl phosphates may also be used. Suitable, but non-limiting species of triaryl phosphates include triphenyl phosphate, tricresyl phosphate, diphenylcresyl phosphate, and diphenylxylyl phosphate, diphenyl(ethylphenyl) phosphate, dicresylphenyl phosphate, and the like. Preferred are those triaryl phosphates wherein at least one aryl group is a monoalkyl-substituted aryl group having one or two carbon atoms in the alkyl group, and preferably one carbon atom in the alkyl group.

The mixed phosphate ester portion of the composition of this disclosure will comprise at least 70% by weight of the total composition, and preferably at least 90% by weight of the total composition.

In another embodiment, the base stock can comprise a mixed alkylaryl phosphate ester such as dibutyl phenyl phosphate, butyl diphenyl phosphate, methyl ethyl phenyl phosphate, and the like. Particularly preferred is dibutyl phenyl phosphate.

In an embodiment, the phosphate ester base stocks used in this disclosure refer to organophosphate esters selected from trialkyl phosphate, dialkyl aryl phosphate, alkyl diaryl phosphate and triaryl phosphate that contain from 3 to 8, preferably from 4 to 5 carbon atoms. Suitable phosphate esters useful in this disclosure include, for example, tri-n-butyl phosphate, tri-isobutyl phosphate, n-butyl di-isobutyl phosphate, di-isobutyl n-butyl phosphate, n-butyl diphenyl phosphate, isobutyl diphenyl phosphate, di-n-butyl phenyl phosphate, di-isobutyl phenyl phosphate, tri-n-pentyl phosphate, tri-isopentyl phosphate, triphenyl phosphate, isopropylated triphenyl phosphates, butylated triphenyl phosphates, and the like. Preferably, the trialkyl phosphate esters are those of tri-n-butyl phosphate and tri-isobutyl phosphate.

Illustrative esters of phosphoric acid which can be employed as base stocks in the compositions of this disclosure are those wherein the groups represented by R¹, R² and R³ are alkyl, alkoxyalkyl, phenyl or substituted phenyl. The preferred base stocks are referred to as phosphates and include trialkyl phosphates, triphenyl and/or substituted phenyl phosphates and mixed phenyl and/or substituted phenyl phosphates. The alkyl groups preferred are those containing from 2 to 12 carbon atoms with the total number of carbon atoms in the trialkyl phosphates being from 12 to 36 carbon atoms. These alkyl groups include, for example, ethyl, propyl, isopropyl, butyl, hexyl, 2-ethylhexyl, dodecyl, decyl, and the like. The substituted phenyl groups represented by R¹, R² and R³ are those containing up to 16 carbon atoms and the alkyl groups contain 1 to 10 carbon atoms provided that the total number of carbon atoms in all of the alkyl groups attached to any one phenyl group be at most 10 carbon atoms.

The alkoxyalkyl groups include, for example, those having the formula

H(C_(n)H_(2n)—O)_(x)—C_(a)H_(2a)—

wherein n is an integer having a value of from 1 to 10, a is an integer having a value of from 2 to 10, and x is an integer of 1 to 10, preferably x is 1. These alkoxylalkyl groups include, for example methoxyethyl, ethoxyethyl, propoxyethyl, ethoxypropyl, ethoxypentyl, propoxydecyl, nonyloxyethyl, octyloxybutyl, and the like. The substituted phenyl groups represented by R¹, R² and R³ include the alkyl-substituted phenyl groups and include, for example, niethylphenyl, ethylphenyl, dimethylphenyl, propylphenyl, nonyjphenyl, decylphenyl, dipentylphenyl, butylhexylphenyl, and the like.

Also included within the definition of the substituted phenyl groups represented by R¹, R² and R³, are, for example, the phenylalkyl phenyl groups (C₆H₅C_(n)H_(2n)—C₆H₄— where n is 1 to 8) containing up to 20 carbon atoms, i.e., cumylphenyl, phenylmethylphenyl (C₆H₅CH₂—C₆H₄—), phenylethylphenyl (C₆H₅—C₂H₄—C₆H₄), phenylpropylphenyl (C₆H₅C₃H₆—C₆H₄—), phenyloctylphenyl (C₆H₅C₈H₁₆—C₆H₄—) and the like; and phenyl-substituted phenyl groups such as o-, m- and p-phenylphenyl (C₆H₅—C₆H₄—), o-, m- and p-methylphenylphenyl (CH₃—C₆H₄—C₆H₄—), o-, m- and p-dimethylphenylphenyl ((CH₃)₂—C₆H₃—C₆H₄—) and the like. Typical examples of these phosphate esters include, for example, dibutylphenyl phosphate, triphenyl phosphate, tricresyl phosphate, tributyl phosphate, tri-2-ethylhexyl, trioctyl phosphate, and mixtures of the above phosphates such as mixture of tributyl phosphate and tricresyl phosphate, mixtures of isooctyl diphenyl phosphate and 2-ethylhexyl diphenyl phosphate, and mixtures of trialkyl phosphates and tricresyl phosphates, and the like. The particularly preferred phosphate esters are those which remain liquid at temperatures of about 30° C.

Preferred phosphate ester base stocks used in this disclosure refer to organophosphate esters selected from trialkyl phosphate, dialkyl aryl phosphate, alkyl diaryl phosphate and triaryl phosphate that contain from 3 to 8, preferably from 4 to 5 carbon atoms. Suitable phosphate esters useful in this disclosure include, for example, tri-n-butyl phosphate, tri-isobutyl phosphate, n-butyl di-isobutyl phosphate, di-isobutyl n-butyl phosphate, n-butyl diphenyl phosphate, isobutyl diphenyl phosphate, di-n-butyl phenyl phosphate, di-isobutyl phenyl phosphate, tri-n-pentyl phosphate, tri-isopentyl phosphate, triphenyl phosphate, isopropylated triphenyl phosphates, and butylated triphenyl phosphates. Preferably, the trialkyl phosphate esters are those of tri-n-butyl phosphate and tri-isobutyl phosphate.

The amounts of each type of phosphate ester in the functional fluid can vary depending upon the type of phosphate ester involved. The amount of trialkyl phosphate in the base stock fluid comprises from about 10 weight percent to about 100 weight percent preferably from about 20 weight percent to about 90 weight percent. The amount weight percent % preferably from 0 weight percent to about 50 weight percent. The amount of alkyl diaryl phosphate in the base stock fluid is typically from 0 weight percent to 30 weight percent, preferably from 0 weight percent to 10 weight percent. The amount of triaryl phosphate in the base stock fluid is typically from 0 weight percent to 20 weight percent and preferably from 0 weight percent to 15 weight percent.

Illustrative base stock fluids useful in the functional fluid compositions of this disclosure are described, for example, in U.S. Pat. Nos. 3,679,587, 3,907,697, 4,206,067, 4,324,674, 5,205,951, 5,464,551, 6,254,799, 6,319,423, 6,649,080, 6,652,772, 6,703,355, 6,764,611, 7,255,808, and 8,227,388, the disclosures of which are incorporated herein by reference. Further, illustrative base stock fluids useful in the functional fluid compositions of this disclosure are described, for example, in U.S. Patent Application Publication Nos. 2002/0179882, 2003/0040443, and 2005/0056809, the disclosures of which are incorporated herein by reference.

Erosion Inhibitors

The functional fluid compositions of this disclosure comprise an erosion inhibitor mixture in an amount effective to inhibit flow-induced electrochemical corrosion. Suitable erosion inhibitors are disclosed, for example, in U.S. Pat. Nos. 5,464,551 and 3,679,587, the disclosures of which are incorporated herein by reference. Preferred erosion inhibitors include the alkali metal salts, and preferably the potassium salt, of a perfluoroalkyl or perfluorocycloalkyl sulfonate as disclosed in U.S. Pat. No. 3,679,587. Such perfluoroalkyl and perfluorocycloalkyl sulfonates preferably encompass alkyl groups of from 1 to 10 carbon atoms and cycloalkyl groups of form 3 to 10 carbon atoms. Several of these perfluoroalkyl sulfonates are available commercially under the tradenames FC-95, FC-98 and the like.

The terms “perhalometallic acid” and “perhalometalloidic acid” as used herein encompass all of the perhalo acids of the metals and metalloids of the Periodic Table capable of forming the perhalo acids. These acids are sometimes referred to as “super acids”. The metals and metalloids which are capable of forming perhalo acids include, for example, beryllium of Group 2A, thorium of Lanthanide series, titanium and zirconium of Group 4B, niobium and tantalum of Group 5B, chromium of Group 6B, manganese and rhenium of Group 7B, iron, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum of Group 8B, gold of Group 1B, aluminum, gallium and boron of Group 3A, phosphorus, arsenic and antimony of Group 5A and tellurium of Group 6A.

Exemplary perhalometallic acids include hexafluoroaluminic, hexafluoroantimonic, hexafluoroferric, hexafluorotitanic, hexafluorothoric, hexafluorogermanic, hexafluorozirconic, tetrafluoroberyllic, trifluorostannous, hexafluorostannic, hexafluoro- and heptafluorotantalic, hexafluorochromic, hexafluoroniobic, hexafluoromanganic, tetrafluoroboric, hexafluoro- and tetrafluorophosphoric, hexafluorosiliconic, hexafluoroarsenic, hexachloroiridic, hexachloroosmic, tetrachloro- and hexachloropalladic, tetrachloro- and hexachloroplatinic, hexachlorodic, hexaidoplatinic and hexabromoiridic, and hexabromoplatinic acids.

The perhalometallate and perhalometalloidate salts can be prepared by reacting the perhalo acid with a basic compound such as an alkali metal base, an alkaline earth metal base, ammonium bases, substituted ammonium bases, phosphonium bases and substituted phosphonium bases. Other basic compounds may also be employed, provided that the basic compound forms a salt with the perhalo acid and such resulting salt is sufficiently soluble in the functional fluid to effect a reduction in the streaming current of the fluid. Exemplary basic compounds include alkali metal hydroxides, oxides and carbonates such as sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium oxide, potassium oxide, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, and the like, alkaline earth metal hydroxides, oxides and carbonates such as calcium hydroxide, barium hydroxide, calcium oxide, barium oxide, calcium carbonate, calcium bicarbonate, and the like. Ammonium hydroxide, substituted quaternary ammonium hydroxides such as tetramethyl ammonium hydroxide, trimethylbenzyl ammonium hydroxide, tetrapropyl ammonium hydroxide, tetrabutyl ammonium hydroxide, and the like, phosphonium hydroxide, tetramethyl phosphonium hydroxide, tetrabutyl phosphonium hydroxide, trimethylbenzyl phosphonium hydroxide, and the like. Other basic compounds include zinc hydroxide, zinc carbonate, zinc bicarbonate, and the like.

The perhalometallate and perhalometalloidate salts may also be prepared by reacting a halogenated metal or metalloid with an ammonium or phosphonium halide or a substituted ammonium or substituted phosphonium halide. Exemplary reactions include a reaction of ammonium fluoride with boron trifluoride, phosphonium fluoride with boron trifluoride, ammonium fluoride with phosphorus pentafluoride, phosphonium chloride with boron trifluoride, tetramethyl ammonium chloride with boron trifluoride, methylbenzyl phosphonium chloride with phosphorus pentafluoride, methylbenzyl phosphonium chloride with boron trichloride, ammonium chloride with antimony pentachloride, triethyl oxonium fluoride with boron trifluoride, triphenyl carbenium chloride with phosphorus pentafluoride, and the like.

A preferred class of perhalometallate and perhalometalloidate salts for use as erosion inhibitors in the functional fluids of this disclosure are believed to have the following general formula:

M[AX_(y)]_(z)

wherein A is a metal or metalloid, and preferably a metalloid; M is a solubilizing cation; X is a halogen, preferably chlorine or fluorine and more preferably fluorine; y is an integer from 3 to 7 and equal to the positive valence of A plus an integer from 1 to 3; and preferably an integer equal to 4 for the metals and metalloids in the second period of the Periodic Table and equal to 6 for the metals and the metalloids in all other periods of the Periodic Table; z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral.

In the above formula, A is an amphoteric metal or metalloid of the type described supra and capable of forming perhalo acids. In a preferred embodiment, A is selected from the group consisting of phosphorus, boron and antimony and, more preferably, phosphorus or boron. M can be any stable cation which imparts significant solubility to the salts in the functional fluid. This solubility is preferably from at least 0.01 gram per liter of the perhalo salt in the functional fluid and, more preferably, at least about 0.01 gram/liter. The preferred solubilizing cation includes alkali metal, ammonium, phosphonium, C₁-C₃₀ hydrocarbyl-substituted ammonium, C₁-C₃₀ hydrocarbyl-substituted phosphonium cations, C₃-C₃₀ trihydrocarbyl carbenium, and C₃-C₃₀ trihydrocarbyl oxonium. The most preferred solubilizing cations are sodium, potassium and ammonium.

Illustrative preferred perhalometallates and perhalometalloidates include, for example, ammonium hexafluorophosphate (NH₄PF₆), N-benzyl-N,N,N-tri methyl ammonium hexafluorophosphate (CH₃)₃(CH₆H₅CH₂)NPF₆, potassium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, ammonium tetrafluoroborate (NH₄BF₄), sodium tetrafluoroborate, zinc tetrafluoroborate, sodium hexafluoroantimonate (NaSbF₆), ammonium hexafluoroantimonate, N-benzyl-N,N,N-triethyl phosphonium hexafluorophosphate, potassium hexafluoroantimonate, and the like.

Methods of preparation of the various perhalometallates and perhalometalloidates are well known in the chemical literature, and many are commercially available.

The concentration of perhalometallate or perhalometalloidate salt in the functional fluid varies, depending upon the salt selected, operating temperature and the like. Generally, however, from 0.0001 to 2 weight percent, preferably from 0.0005 to 1 weight percent, of the salt is incorporated into the functional fluid, and more preferably from 0.001 to 0.01 weight percent.

In an embodiment, the erosion inhibitors employed in the compositions of this disclosure are the alkali metal salts of perfluoroalkyl sulfonic acids and have the general formula

R₁SO₃M

where M is an alkali metal, for example sodium, lithium, potassium, rubidium or cesium and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18. These salts are, for example, potassium perfluoromethane sulfonate, potassium perfluoroethane sulfonate, sodium perfluorobutane sulfonate, sodium perfluorocyclohexane sulfonate, potassium perfluorooctane sulfonate, cesium perfluorooctadecane sulfonate, potassium perfluorocyclopentane sulfonate, potassium perfluoropentane sulfonate, and the like. It is preferred that the perfluorinated anionic surfactants contain at least 5 carbon atoms and especially preferred that they contain from 7 to 12 carbon atoms. Other perfluorinated anionic surfactants which can be employed in the compositions of this disclosure are the alkali metal salts of perfluorinated alkyl disulfonic acids and the like. These disulfonic acid salts are, for example, dipotassium perfluorocyclohexane disulfonic C₅F₁₀(SO₃K)₂, dipotassium bis(perfluorocyclohexane sulfonate) KO₃S—C₅F₁₀—SO₃K and the like.

The perfluorinated alkyl groups represented by R₁ include perfluorinated alkyl groups and perfluorinated cycloalkyl groups, for example, perfluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl, perfluoroheptyl, perfluorooctyl, perfluorodecyl, perfluorooctodecyl, and the like. The cycloalkyl groups are, for example, perfluorocyclopentyl, perfluorocyclohexyl, perfluorocycloheptyl, perfluoro(ethylcyclohexyl), perfluoro(cyclohexylmethyl), perfluoro(cyclohexylethyl), perfluoro(cyclohexylpropyl), pirfluoro(methylcyclohexyl), perfluoro(dimethylcyclohexyl), and the like. It is preferred that the perfluoroalkyl group contain at least 5 carbon atoms and even more preferred that it contain 7 or more carbon atoms.

Referring to the above formula, M is an alkali metal such as lithium, sodium, potassium, or cesium. M can also be calcium or barium. It can also be ammonium (NH₄). The R₁ group consists of a perfluorinated hydrocarbyl group from 1 to 12 carbon atoms, preferably C1 to C8. Low molecular weight metal salt of perfluoroalkyl sulfonic acid are soluble in the phosphate ester base stock. The hydrocarbyl group can be linear or branched. A preferred example is metal salts of 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonic acid. M is preferably an alkali metal and most preferably potassium.

Illustrative erosion inhibitors include, for example, potassium trifluoromethanesulfonate or potassium triflate; lithium trifluoromethanesulfonate or lithium triflate; potassium 1,1,2,2,2-pentafluoroethane-1-sulfonate-; potassium 1,1,2,2,3,3,3-heptafluoropropane-1-sulfonate-; potassium 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonate-; potassium 1,1,2,2,3,3,4,4,5,5,5-undecafluoropentane-1-sulfonate-; potassium 1,1,2,2,3,3,4,4,5,5,6,6,6-tridecafluorohexane-1-sulfonate-; potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,7-pentadecafluoroheptane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctane-1-sulfonate; and the like.

The foregoing additives are readily prepared by neutralization of the corresponding acid (i.e., a compound of the above formula except that M is H) with an alkali metal hydroxide or quaternary ammonium hydroxide. Additives of the foregoing formula are also commercially available compounds.

The anti-erosion additive is incorporated in the phosphate ester base stock in an amount sufficient to enhance the anti-erosive properties of the fluid. Typically the addition comprises from about 0.01 weight percent to about 0.5 weight percent based on the weight of the base stock.

In another embodiment, the erosion inhibitor mixtures of this disclosure can include a salt represented by the formula

R₁CH₂CH₂SO₃M

where R₁═F(CH₂CF₂)_(y), y=1 to about 9 and M is an alkali metal selected from lithium, sodium, potassium, rubidium or cesium. M can also be a quaternary ammonium cation having following formula RR′R″R″′N+ (wherein R, R′, R″, R′″ are independently hydrogen, hydrocarbyl groups of from 1 to 30 carbon atoms and oxygen containing hydrocarbyl groups of from 1 to 25 carbon atoms.

The foregoing additives are readily prepared by neutralization of the corresponding acid (i.e., a compound of the above formula except that M is H) with an alkali metal hydroxide or quaternary ammonium hydroxide. Additives of the foregoing formula are also commercially available compounds.

The anti-erosion additive is incorporated in the phosphate ester base stock in an amount sufficient to enhance the anti-erosive properties of the fluid. Typically the addition comprises from about 0.01 weight percent to about 0.5 weight percent based on the weight of the base stock.

Illustrative erosion inhibitors useful in the erosion inhibitor mixtures of this disclosure are described, for example, in U.S. Pat. Nos. 3,679,587, 3,907,697, 4,206,067, 4,324,674, 5,205,951, 5,464,551, 6,254,799, 6,319,423, 6,649,080, 6,652,772, 6,703,355, 6,764,611, 7,255,808, and 8,227,388, the disclosures of which are incorporated herein by reference. Further, illustrative erosion inhibitors useful in the erosion inhibitor mixtures of this disclosure are described, for example, in U.S. Patent Application Publication Nos. 2002/0179882, 2003/0040443, and 2005/0056809, the disclosures of which are incorporated herein by reference.

The erosion inhibitor mixture is, for example, employed in an amount effective to inhibit erosion in the power transmission mechanisms of an aircraft and, preferably, is employed in an amount of from about 0.01 to about 0.15 weight percent, based on the total weight of the functional fluid composition and more preferably from about 0.02 to about 0.1 weight percent.

Other Additives

The functional fluids (e.g., hydraulic fluids) of this disclosure contain from 1 weight percent to 20 weight percent based on total weight composition of additives selected from one or more antioxidants, acid scavengers, viscosity index (VI) improvers, rust inhibitors, antifoamers, corrosion inhibitors, and the like. The use of those conventional additives provides satisfactory hydrolytic, oxidative stability and viscometric properties of the functional fluid compositions under normal and severe conditions found, for example, in aircraft hydraulic systems.

The types and quantities of performance additives used in combination with the instant disclosure in functional fluid compositions are not limited by the examples shown herein as illustrations.

Antioxidants

The functional fluid compositions used in this disclosure can further optionally comprise an antioxidant or mixture of antioxidants in an amount effective to inhibit oxidation of the functional fluid or any of its components.

Antioxidants useful in functional fluid compositions in this disclosure include, for example, polyphenols, trialkylphenols and di(alkylphenyl) amines, examples of which include bis (3,5-di-tert-butyl-4-hydroxyphenyl) methane, 1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxyphenyl) benzene, 2,6-di-tert-butyl-4-methylph-enol, tetrakis (methylene (3,5-di-tert-butyl-4-hydroxyhydrocinnamate) methane, di(n-octylphenyl) amine, and the like. Typical amounts for each type of antioxidants can be from about 0.1 weight percent to 2 weight percent.

Other representative antioxidants include, by way of example, phenolic antioxidants, such as 2,6-di-tert-buty-p-cresol, tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]-methane (Irganox™ 1010), bis (3,5 di-tert-butyl-4 hydroxyphenyl) methane (Ethanox 702), 1,3,5-trimethyl-2,4,6-tris (3,5-di-tert butyl-4-hydroxyphenyl) benzene (Ethanox 330) and the like; amine antioxidants including, by way of example, diarylamines, such as octylated diphenyl amine (Vanlube™ 81), phenyl-α-naphthylamine, alkylphenyl-α-naphthylamine, or the reaction product of N-phenylbenzylamine with 2,4,4-trimethylpentene (Irganox™ L-57), diphenylamine, ditoylamine, phenyl tolyamine, 4,4′-diaminodiphenylamine, di-p-methoxydiphenylamine, or 4-cyclohexylaminodiphenylamine. Still other suitable antioxidants include aminophenols such as N-butylaminophenol, N-methyl-N-amylaminophenol and N-isooctyl-p-aminophenol, as well as mixtures of any such antioxidants.

A preferred mixture of antioxidants comprises 2,6-di-tert-butyl-p-cresol and di(octylphenyl)amine (e.g., a 1:1 mixture). Another preferred mixture of antioxidants is 2,6-di-tert-butyl-p-cresol, di(octylphenyl)amine and 6-methyl-2,4-bis(octylthio)-methyl]-phenol (e.g., 1:2:4 mixture). Still another preferred mixture of antioxidants is 2,6-di-tert-butyl-p-cresol, di(octylphenyl)amine and tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane (e.g., a 1:2:3 mixture).

The antioxidant or mixture of antioxidants is employed in an amount effective to inhibit oxidation of the functional fluid. The antioxidant or mixture of antioxidants is employed in an amount ranging from about 0 to about 3 weight percent, more preferably from about 0.5 to 2.5 weight percent and still more preferably at from about 1 to 2 weight percent based on the total weight of the functional fluid composition.

Antioxidants retard the oxidative degradation of base fluids during service. Such degradation may result in deposits on metal surfaces, the presence of sludge, or a viscosity increase in the functional fluid. One skilled in the art knows a wide variety of oxidation inhibitors that are useful in functional fluid compositions.

Useful antioxidants include hindered phenols. These phenolic antioxidants may be ashless (metal-free) phenolic compounds or neutral or basic metal salts of certain phenolic compounds. Typical phenolic antioxidant compounds are the hindered phenolics which are the ones which contain a sterically hindered hydroxyl group, and these include those derivatives of dihydroxy aryl compounds in which the hydroxyl groups are in the o- or p-position to each other. Typical phenolic antioxidants include the hindered phenols substituted with C₆+ alkyl groups and the alkylene coupled derivatives of these hindered phenols. Examples of phenolic materials of this type 2-t-butyl-4-heptyl phenol; 2-t-butyl-4-octyl phenol; 2-t-butyl-4-dodecyl phenol; 2,6-di-t-butyl-4-heptyl phenol; 2,6-di-t-butyl-4-dodecyl phenol; 2-methyl-6-t-butyl-4-heptyl phenol; and 2-methyl-6-t-butyl-4-dodecyl phenol. Other useful hindered mono-phenolic antioxidants may include for example hindered 2,6-di-alkyl-phenolic proprionic ester derivatives. Bis-phenolic antioxidants may also be advantageously used in combination with the instant disclosure. Examples of ortho-coupled phenols include: 2,2′-bis(4-heptyl-6-t-butyl-phenol); 2,2′-bis(4-octyl-6-t-butyl-phenol); and 2,2′-bis(4-dodecyl-6-t-butyl-phenol). Para-coupled bisphenols include for example 4,4′-bis(2,6-di-t-butyl phenol) and 4,4′-methylene-bis(2,6-di-t-butyl phenol).

Effective amounts of one or more catalytic antioxidants may also be used. The catalytic antioxidants comprise an effective amount of a) one or more base fluid soluble polymetal organic compounds; and, effective amounts of b) one or more substituted N,N′-diaryl-o-phenylenediamine compounds or c) one or more hindered phenol compounds; or a combination of both b) and c). Catalytic antioxidants are more fully described in U.S. Pat. No. 8,048,833, herein incorporated by reference in its entirety.

Non-phenolic oxidation inhibitors which may be used include aromatic amine antioxidants and these may be used either as such or in combination with phenolics. Typical examples of non-phenolic antioxidants include: alkylated and non-alkylated aromatic amines such as aromatic monoamines of the formula R⁹R⁹R¹⁰N where R⁸ is an aliphatic, aromatic or substituted aromatic group, R⁹ is an aromatic or a substituted aromatic group, and R¹⁰ is H, alkyl, aryl or R¹¹S(O)_(X)R¹² where R¹¹ is an alkylene, alkenylene, or aralkylene group, R¹² is a higher alkyl group, or an alkenyl, aryl, or alkaryl group, and x is 0, 1 or 2. The aliphatic group R⁸ may contain from 1 to 20 carbon atoms, and preferably contains from 6 to 12 carbon atoms. The aliphatic group is an aliphatic group. Preferably, both R⁸ and R⁹ are aromatic or substituted aromatic groups, and the aromatic group may be a fused ring aromatic group such as naphthyl. Aromatic groups R⁸ and R⁹ may be joined together with other groups such as S.

Typical aromatic amines antioxidants have alkyl substituent groups of at least 6 carbon atoms. Examples of aliphatic groups include hexyl, heptyl, octyl, nonyl, and decyl. Generally, the aliphatic groups will not contain more than 14 carbon atoms. The general types of amine antioxidants useful in the present compositions include diphenylamines, phenyl naphthylamines, phenothiazines, imidodibenzyls and diphenyl phenylene diamines. Mixtures of two or more aromatic amines are also useful. Polymeric amine antioxidants can also be used. Particular examples of aromatic amine antioxidants useful in the present disclosure include: p,p′-dioctyldiphenylamine; t-octylphenyl-alpha-naphthylamine; phenyl-alphanaphthylamine; and p-octylphenyl-alpha-naphthylamine.

Sulfurized alkyl phenols and alkali or alkaline earth metal salts thereof also are useful antioxidants.

Preferred antioxidants include hindered phenols, arylamines. These antioxidants may be used individually by type or in combination with one another. Such additives may be used in an amount of 0.01 to 5 weight percent, preferably 0.01 to 1.5 weight percent, more preferably zero to less than 1.5 weight percent, more preferably zero to less than 1 weight percent.

Acid Scavengers

Acid scavengers useful in functional fluid compositions of this disclosure to neutralize phosphoric acid and dialkyl phosphoric acid produced from the hydrolysis and thermal degradation of the phosphate ester base stocks. Examples of acid scavengers include epoxy compounds such as epoxycyclohexane carboxylates. Typical amounts that can be used as acid scavenger can be from about 1 to about 10 weight percent based on the total weight of functional fluid.

The functional fluid compositions (e.g., aircraft hydraulic fluid compositions) of this disclosure further comprise an acid control additive, acid receptor, or acid scavenger in an amount sufficient to neutralize acids formed in functional fluid, such as phosphoric acid and its partial esters. Suitable acid control additives are described, for example, in U.S. Pat. Nos. 5,464,551, 3,723,320 and 4,206,067, the disclosures of which are incorporated herein by reference.

In an embodiment, preferred acid control additives have the formula:

where R is selected from the group consisting of an alkyl group of from 1 to 10 carbon atoms optionally containing from 1 to 4 ether oxygen atoms therein and cycloalkyl of from 3 to 10 carbon atoms, each R′ is independently selected from the group consisting of hydrogen, alkyl of from 1 to 10 carbon atoms and —C(O)OR″ where R″ is alkyl of from 1 to 10 carbon atoms optionally containing from 1 to 4 ether oxygen atoms therein or cycloalkyl of from 3 to 10 carbon atoms, R′″ is selected from the group consisting of hydrogen, alkyl of from 1 to 10 carbons atoms and —C(O)OR″ where R″ is alkyl of from 1 to 10 carbon atoms optionally containing from 1 to 4 ether oxygen atoms therein or cycloalkyl of from 3 to 10 carbon atoms.

Preferred acid scavengers of the above formula are the monoepoxide 7-oxabicyclo[4.1.0]heptane-3-carboxylic acid, 2-ethylhexyl ester which is disclosed in U.S. Pat. No. 3,723,320, and the monoepoxide 7-oxa-bicyclo[4.1.0]-heptane-3,4-dicarboxylic acid, dialkyl esters (e.g., the di-isobutyl ester). Dialkyl esters of this monoepoxide are also disclosed in U.S. Pat. No. 3,723,320. The trialkyl and tetraalkyl esters are prepared via conventional Diels-Alder reaction procedures via a suitable unsaturated trialkyl or tetraalkyl ester and a suitable 1,3-diene. The Diels-Alder reaction provides for 4+2 cyclo addition to provide for a cyclohexene derivative having the suitable trialkyl or tetraalkyl esters. The unsaturation in the cyclohexane is utilized to provide for epoxide formation via conventional methods.

Suitable unsaturated trialkyl and tetraalkyl esters are known in the art. For example, tetraethyl ethylene tetracarboxylate is commercially available. The alkyl groups of this tetraethyl ester can readily be exchanged via conventional techniques to provide for other esters as described above.

The use of such di-, tri- and tetraalkyl esters of this monoepoxide provide for enhanced seal compatibility for the functional fluids of this disclosure as well as with conventional formulations employing conventional trihydrocarbyl phosphate base stocks with the ethylene propylene seals used, for example, in aircraft hydraulic systems.

Also useful are diepoxides such as those disclosed in U.S. Pat. No. 4,206,067 which contain two linked cyclohexane groups to each of which is fused an epoxide group. Such diepoxide compounds correspond to the formula

wherein R³ is an organic group containing 1 to 10 carbon atoms, from 0 to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R⁴ through R⁹ are independently selected from among hydrogen and aliphatic groups containing 1 to 5 carbon atoms. Exemplary diepoxides include 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane, bis (3,4-epoxy-6-methylcyclohexylmethyl adipate), 2-(3,4-epoxycyclohexyl)-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane. The concentration of the acid scavenger in the fluid composition is preferably between about 1.5% and about 10%, more preferably between about 2% and about 8% by weight, which is generally sufficient to maintain the hydraulic fluid in a serviceable condition for up to approximately 3000 hours of aircraft operation.

Illustrative diepoxides include, for example, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane, bis (3,4-epoxy-6-methylcyclohexylmethyl adipate), 2-(3,4-epoxycyclohexyl)-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane, and the like.

The concentration of the acid scavenger in the fluid composition is preferably between about 1.5 weight percent and about 10 weight percent, more preferably between about 2 weight percent and about 8 weight percent by weight, which is generally sufficient to maintain the hydraulic fluid in a serviceable condition for up to approximately 3000 hours of aircraft operation.

The acid control additive whether as mono- di-, tri- or tetra-ester is employed in an amount effective to scavenge the acid generated, typically as partial esters of phosphoric acid, during operation of the power transmission mechanisms of an aircraft. Preferably, the acid control additive is employed in an amount ranging from about 4 to about 10 weight percent, based on the total weight of the hydraulic fluid composition, and more preferably from 5 to 9 weight percent and still more preferably from 6 to 8 weight percent.

Viscosity Index Improvers

A viscosity index (VI) improver is employed in the functional fluid compositions of this disclosure in an amount effective to reduce the effect of temperature on the viscosity of the functional fluid, e.g., aircraft hydraulic fluid. Examples of suitable VI improvers are disclosed, for example, in U.S. Pat. Nos. 5,464,551 and 3,718,596, the disclosures of which are incorporated herein by reference. Preferred VI improvers include poly (alkyl acrylate) and poly (alkyl methacrylate) esters of the type disclosed in U.S. Pat. No. 3,718,596 and which are commercially available (e.g., available under the tradenames HF-411, HF-460, PA-7570, and the like). Such esters typically have a weight average molecular weight range of from about 50,000 to about 1,500,000 and preferably from about 50,000 to 250,000. Preferred VI improvers include those having a molecular weight peak at about 70,000 to 100,000 (e.g., about 85,000 or 90,000 to 100,000). Mixtures of VI improvers can also be used.

The VI improver is present in an amount effective to reduce the effect of temperature on viscosity, preferably from about 2 to about 10 weight percent (on an active ingredient) and more preferably from about 4 to about 6 weight percent based on the total weight of the functional fluid composition. The VI improver can be mixed with a portion of the phosphate ester base stock, typically as a 1:1 mixture and then added to the balance of the functional fluid.

Viscosity index improvers (also known as VI improvers, viscosity modifiers, and viscosity improvers) can be included in the functional fluid compositions of this disclosure.

Viscosity index improvers provide functional fluids with high and low temperature operability. These additives impart shear stability at elevated temperatures and acceptable viscosity at low temperatures.

Suitable viscosity index improvers include high molecular weight hydrocarbons, polyesters and viscosity index improver dispersants that function as both a viscosity index improver and a dispersant. Typical molecular weights of these polymers are between about 10,000 to 1,500,000, more typically about 20,000 to 1,200,000, and even more typically between about 50,000 and 1,000,000. The typical molecular weight for polymethacrylate or polyacrylate viscosity index improvers is less than about 50,000.

Examples of suitable viscosity index improvers are linear or star-shaped polymers and copolymers of methacrylate, butadiene, olefins, or alkylated styrenes. Polyisobutylene is a commonly used viscosity index improver. Another suitable viscosity index improver is polymethacrylate (copolymers of various chain length alkyl methacrylates, for example), some formulations of which also serve as pour point depressants. Other suitable viscosity index improvers include copolymers of ethylene and propylene, hydrogenated block copolymers of styrene and isoprene, and polyacrylates (copolymers of various chain length acrylates, for example). Specific examples include styrene-isoprene or styrene-butadiene based polymers of 50,000 to 200,000 molecular weight.

Olefin copolymers, are commercially available under the trade designation “PARATONE®” (such as “PARATONE® 8921” and “PARATONE® 8941”); under the trade designation “HiTEC®” (such as “HiTEC® 5850B”; and under the trade designation “Lubrizol® 7067C”. Hydrogenated polyisoprene star polymers are commercially available from Infineum International Limited, e.g., under the trade designation “SV200” and “SV600”. Hydrogenated diene-styrene block copolymers are commercially available from Infineum International Limited, e.g., under the trade designation “SV 50”.

Other illustrative viscosity index improvers in this disclosure include, for example, polymethacrylate or polyacrylate polymers, including dispersant polymethacrylate and dispersant polyacrylate polymers. These polymers offer significant advantages in solubility in esters of a non-aromatic dicarboxylic acid, preferably alkyl adipate esters. The polymethacrylate or polyacrylate polymers can be linear polymers which are available from Evnoik Industries under the trade designation “Viscoplex®” (e.g., Viscoplex 6-954) or star polymers which are available from Lubrizol Corporation under the trade designation Asteric™ (e.g., Lubrizol 87708 and Lubrizol 87725).

In an embodiment of this disclosure, the viscosity index improvers may be used in an amount of from 1.0 to about 20% weight percent, preferably 5 to about 15 weight percent, and more preferably 8.0 to about 12 weight percent, based on the total weight of the functional fluid.

As used herein, the viscosity index improver concentrations are given on an “as delivered” basis. Typically, the active polymer is delivered with a diluent fluid. The “as delivered” viscosity index improver typically contains from 20 weight percent to 75 weight percent of an active polymer for polymethacrylate or polyacrylate polymers, or from 8 weight percent to 20 weight percent of an active polymer for olefin copolymers, hydrogenated polyisoprene star polymers, or hydrogenated diene-styrene block copolymers, in the “as delivered” polymer concentrate.

Rust Inhibitors

The functional fluid compositions of this disclosure optionally comprise a rust inhibitor or a mixture of rust inhibitors in an amount effective to reduce the formation of rust or corrosion on metal surfaces exposed to the functional fluid. Suitable rust inhibitors are described in U.S. Pat. Nos. 5,035,084, 4,206,067 and 5,464,551, the disclosures of which are incorporated herein by reference. Representative rust inhibitors include, by way of example, calcium dinonylnaphthalene sulfonate, a Group I or Group II metal overbased and/or sulfurized phenate, sulfonate or carboxylate, a compound of the formula:

R⁴N[CH₂CH(R⁵)OH]₂

wherein R⁴ is selected from the group consisting of alkyl of from 1 to 40 carbon atoms, —COOR⁶ and —CH₂CH₂N[CH₂CH(R⁵)OH]₂ where R⁶ is alkyl of from 1 to 40 carbon atoms, and each R⁵ is independently selected from the group consisting of hydrogen and methyl, including N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylene diamine and N,N-bis(2-hydroxyethyl)tallow amine (e.g., N tallow amine alkyl-2,2′-iminobisethanol, sold under the trade name Ethomeen T/12).

The Group I and Group II metal overbased and/or sulfurized phenates, preferably are either sulfurized Group I or Group II metal phenates (without CO₂ added) having a Total Base Number (TBN) of from greater than 0 to about 200 or a Group I or Group 11 metal overbased sulfurized phenate having a TBN of from 75 to 400 prepared by the addition of carbon dioxide during the preparation of the phenate. More preferably, the metal phenate is a potassium or calcium phenate. The phenate advantageously modifies the pH to provide enhanced hydrolytic stability.

Each of these components are either commercially available or can be prepared by art recognized methods. For example, Group II metal overbased sulfurized phenates are commercially available under the trade name OLOA® including, OLOA 219®, OLOA 216Q® and the like and are described in U.S. Pat. No. 5,318,710 and U.S. Pat. No. 4,206,067. Likewise, N,N,N′,N′-tetrakis(2-hydroxy-propyl)ethylenediamine is disclosed in U.S. Pat. No. 4,324,674. The disclosures of each of these patents are incorporated herein by reference.

Group I or II metal dinonylnaphthalene sulfonates, such as calcium dinonylnaphthalene sulfonate (Na-Sul 729 commercially available) may also be used as a rust inhibitor in the hydraulic fluid composition in an amount ranging from 0.2 to 1.0 weight percent of the functional fluid composition.

The rust inhibitor or mixture of rust inhibitors is employed in an amount effective to inhibit the formation of rust. The rust inhibitor is employed in an amount ranging from about 0 to about 1 weight percent, preferably 0.001 to about 1 weight percent more preferably about 0.005 to about 0.5 weight percent, and still more preferably at about 0.01 to 0.1 weight percent based on the total weight of the functional fluid composition. In an embodiment, the rust inhibitor comprises a mixture of N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine and a Group II metal overbased phenate (e.g., a 5:1 mixture). In another embodiment, the rust inhibitor comprises a mixture of N,N-bis(2-hydroxyethyl)tallow amine (Ethomeen®T/12) and a Group II metal overbased phenate (e.g., a 5:1 mixture).

Antifoam Agents

Anti-foam agents may advantageously be added to functional fluid compositions. These agents retard the formation of stable foams. Silicones and organic polymers are typical anti-foam agents. For example, polysiloxanes, such as silicon oil or polydimethyl siloxane, provide antifoam properties. Anti-foam agents are commercially available and may be used in conventional minor amounts along with other additives such as demulsifiers; usually the amount of these additives combined is less than 1 weight percent and often less than 0.1 weight percent.

Corrosion Inhibitors

Corrosion inhibitors are additives that protect lubricated metal surfaces against chemical attack by water or other contaminants. A wide variety of these are commercially available.

One type of corrosion inhibitor additive is a polar compound that wets the metal surface preferentially, protecting it with a film of oil. Another type of corrosion inhibitor additive absorbs water by incorporating it in a water-in-oil emulsion so that only the oil touches the metal surface. Yet another type of corrosion inhibitor additive chemically adheres to the metal to produce a non-reactive surface. Examples of suitable additives include zinc dithiophosphates, metal phenolates, basic metal sulfonates, fatty acids and amines. Such additives may be used in an amount of about 0.01 to 5 weight percent, preferably about 0.01 to 1.5 weight percent.

When functional fluid compositions contain one or more of the additives discussed above, the additive(s) are blended into the composition in an amount sufficient for it to perform its intended function. Typical amounts of such additives useful in the present disclosure are described herein.

It is noted that many of the additives are shipped from the additive manufacturer as a concentrate, containing one or more additives together, with a certain amount of base oil diluents. Accordingly, the weight amounts in the table below, as well as other amounts mentioned herein, are directed to the amount of active ingredient (that is the non-diluent portion of the ingredient). The weight percent (wt %) indicated below is based on the total weight of the functional fluid composition.

The functional fluid compositions of this disclosure can optionally contain further additives such as copper corrosion inhibitors (e.g., thiadiazoles, triazoles, triazole derivatives such as benzo or alkyl benzo triazole), dyes, etc. Such additives are well-known in the art and are commercially available.

Illustrative additives useful in the functional fluid compositions of this disclosure are described, for example, in U.S. Pat. Nos. 3,679,587, 3,907,697, 4,206,067, 4,324,674, 5,205,951, 5,464,551, 6,254,799, 6,319,423, 6,649,080, 6,652,772, 6,703,355, 6,764,611, 7,255,808, and 8,227,388, the disclosures of which are incorporated herein by reference. Further, illustrative additives useful in the functional fluid compositions of this disclosure are described, for example, in U.S. Patent Application Publication Nos. 2002/0179882, 2003/0040443, and 2005/0056809, the disclosures of which are incorporated herein by reference.

The functional fluids of this disclosure are useful as aircraft hydraulic fluids and the like. The aircraft hydraulic fluid compositions described herein are useful in aircraft where they operate as a power transmission medium. The components of the functional fluid compositions (e.g., aircraft hydraulic fluid compositions) interact synergistically and the selection of the ratio of the erosion inhibitor mixture content of the fluid is essential to providing an unexpected and surprising balance of combined erosion inhibition effectiveness properties and other properties critical to aviation hydraulic oils, including acceptable hydrolytic stability, high flash point, good anti-wear properties, acceptable erosion protection, acceptable low temperature flow properties (viscosity), and elastomer compatibility.

In particular, orifices in the servo control valves of aircraft hydraulic systems are subject to erosion which is attributed to streaming current induced by fluid flow. Valve orifice erosion, if extensive, can greatly impair the functioning of the valve as a precise control mechanism. The functional fluids of this disclosure provide effective erosion inhibition in aircraft hydraulic systems.

The foregoing additives are all commercially available materials. These additives may be added independently but are usually precombined in packages which can be obtained from suppliers of functional fluid additives. Additive packages with a variety of ingredients, proportions and characteristics are available and selection of the appropriate package will take the requisite use of the ultimate composition into account.

The following non-limiting examples are provided to illustrate the disclosure.

EXAMPLES Example 1

Tributyl phosphate base fluid was used as the poor reference and the commercial potassium perfluorooctyl sulfonate salt (PFOS) was used as the good reference in FIG. 1. The fluids (Entries 1-1 and 1-2) were tested for conductivity (ASTM D2624), copper corrosion (ASTM D130), acid numbers (ASTM D974), and Karl Fisher water content (ASTM D6304). As shown in Entry 1-1 in FIG. 1 with just the tributyl phosphate base fluid, almost no conductivity was detected, although the TAN (D974) and copper corrosion (D130-7) results are good. However, when 0.048 wt % PFOS was added to the base fluid (Entry 2), the conductivity increased to 0.95 mho/cm while both the TAN and copper corrosion were maintained as low as the Entry 1-1. This illustrates that when a PFOS anti-erosion additive is added to the fluid, the resulting fluid possesses good conductivity while maintaining sound copper corrosion protection as well as good acidity control, thereby providing proof of no harm to the other critical fluid properties which are sensitive to the presence of anti-erosion additives.

While PFOS has contributed satisfactory performance attributes to aviation hydraulic fluid for many years, the limited supply issue forces fluid providers to seek alternate supply of different anti-erosion additives to replace PFOS. Examples 2-4 below illustrate the evaluation of using single anti-erosion additive versus the use of combinations of two anti-erosion additives, namely potassium hexafluorophosphate and potassium trifluoromethanesulfonate. Clearly as shown in the Example 4, several combinations exhibit strong synergistic effect and the results are totally unexpected when compared to the use of a single additive in Examples 2 and 3. It has been found that the presence of two different kinds of anti-erosion additives can synergistically help with each other to achieve this unexpected performance.

Evaluation of KPF₆ and KSO₃CF₃ as Single Anti-Erosion Additive and Combinations of KPF₆/KSO₃CF₃ Using the Methodology

Example 2

Potassium hexafluorophosphate (KPF₆) was blended at different concentrations in tributyl phosphate as shown in FIG. 2. The blends (Entries 2-1 through 2-8) were tested for conductivity (ASTM D2624), copper corrosion (ASTM D130), acid numbers (ASTM D974), and Karl Fisher water content (ASTM D6304). Elemental analysis (i.e., potassium content) was determined in accordance with ASTM D5185 and ASTM D4951. The results are shown in FIG. 2. The electric conductivity data is higher than the preferred range except for Entry 1, but the copper strip corrosivity and the TAN increase levels are acceptable across the wide range. However, the Karl Fisher data indicates that none of the KPF₆ concentrations achieved the low water content as desired and it doesn't seem to have a predictable pattern of acceptable water content versus concentration.

Example 3

Potassium trifluoromethanesulfonate (KSO₃CF₃) was blended at different concentrations in tributyl phosphate as shown in FIG. 3. The blends (Entries 3-1 through 3-6) were tested for conductivity (ASTM D2624), copper corrosion (ASTM D130), acid numbers (ASTM D974), and Karl Fisher water content (ASTM D6304). Elemental analysis (i.e., potassium content) was determined in accordance with ASTM D5185 and ASTM D4951. The results are shown in FIG. 3. The electric conductivity data is only acceptable at the concentration<0.05% to be in the preferred range, but the copper strip corrosivity and the TAN increase levels are widely acceptable up to 0.07%. However, the Karl Fisher data indicates that only at 0.04% concentration was below the acceptable water content.

Example 4

Potassium hexafluorophosphate (KPF₆) and potassium trifluoromethanesulfonate (KSO₃CF₃) were blended together at different concentrations in tributyl phosphate as shown in FIG. 4. The blend mixtures (Entries 4-1 through 4-6) were tested for conductivity (ASTM D2624), copper corrosion (ASTM D130), acid numbers (ASTM D974), and Karl Fisher water content (ASTM D6304). Elemental analysis (i.e., potassium content) was determined in accordance with ASTM D5185 and ASTM D4951. The results are shown in FIG. 4. A strong synergistic effect is shown in Entry 4-3 and Entry 4-4. The water content of the mixture anti-erosion additive samples is lower than using either of the additives alone while the other performance features (TAN, copper and conductivity) are also satisfactory. The synergies fall in the mid-range of the concentration curve where neither additive component is present in a dominant concentration and both need to work together to achieve the high effectiveness. A range of potassium hexafluorophosphate (KPF₆) from 0.002% to 0.005% (i.e. 0.002%<[wt % KPF₆]<0.005%) and a range of potassium trifluoromethanesulfonate (KSO₃CF₃) from 0.020% to 0.032% (i.e. 0.032%>[wt % KSO₃CF₃]>0.020%) maximizes the synergistic effect.

Evaluation of KSO₃C₄F₉ as a Single Anti-Erosion Additive and Combinations of KPF₆/KSO₃C₄F₉ Using the Methodology

Example 5

Potassium perfluorobutanesulfonate (KSO₃C₄F₉) was blended at different concentrations in tributyl phosphate as shown in FIG. 5. The blends (5-A through 5-F) were tested for conductivity (ASTM D2624), copper corrosion (ASTM D130), acid numbers (ASTM D974), and Karl Fisher water content (ASTM D6304). Elemental analysis (i.e., potassium content) was determined in accordance with ASTM D5185 and ASTM D4951. The results are shown in FIG. 5. The electric conductivity data is only acceptable at the concentration equal or <0.08% to fall in the preferred range but the copper strip corrosivity and the TAN increase levels are widely acceptable up to 0.20%. However, the Karl Fisher data indicates that the anti-erosion additive only at either 0.03% (Entry 5-A) or 0.05% concentration (Entry 5-B) to possess an acceptable water content.

Example 6

Potassium hexafluorophosphate (KPF₆) and potassium perfluorobutanesulfonate (KSO₃C₄F₉) were blended together at different concentrations in tributyl phosphate as shown in FIG. 6. The blend mixtures (Entries 6-1 through 6-6) were tested for conductivity (ASTM D2624), copper corrosion (ASTM D130), acid numbers (ASTM D974), and Karl Fisher water content (ASTM D6304). Elemental analysis (i.e., potassium content) was determined in accordance with ASTM D5185 and ASTM D4951. The results are shown in FIG. 6. None of the mixtures show any synergies as shown in FIG. 6. The interactions of these two anti-erosion additives are not in favor of reducing water resistance, which is opposite to what was observed in the mixtures of KPF₆ and KSO₃CF₃.

Example 7

Trifluoromethane sulfonic acid zinc salt and trifluoromethane sulfonic acid calcium salt were blended at different concentrations in tributyl phosphate as shown in FIG. 7. The blend mixtures (7-1 through 7-6) were tested for conductivity (ASTM D2624), copper corrosion (ASTM D130), and acid numbers (ASTM D974). Elemental analysis (i.e., zinc and calcium content) was determined in accordance with ASTM D5185 and ASTM D4951. The results are shown in FIG. 7. Not all metal salts behave well in the copper corrosion test. As shown in FIG. 7, zinc trifluoromethane sulfonate performs poorly in the copper corrositivity ASTM D130 test.

FIG. 8 shows various classifications of copper corrosion tests in accordance with ASTM D130.

PCT and EP Clauses:

1. A functional fluid composition comprising:

(a) a fluid base stock as a major component, wherein the fluid base stock comprises one or more phosphate ester fluid base stocks represented by the formula

wherein R¹, R² and R³ are independently a substituted or unsubstituted alkyl or aryl group; and

(b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises at least a first erosion inhibitor and a second erosion inhibitor; wherein the first erosion inhibitor comprises a perhalometallate or perhalometalloidate salt represented by the formula:

M[AX_(y)]_(z)

wherein A is a metal or metalloid; M is a solubilizing cation; X is a halogen; y is an integer from 1 to 7 and equal to the positive valence of A; and z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral; and wherein the second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid represented by the formula

R₁SO₃M

where M is an alkali metal, and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18; and wherein erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.

2. The functional fluid composition of clause 1 wherein the one or more phosphate ester fluid base stocks comprise trialkyl phosphate, dialkyl aryl phosphate, alkyl diaryl phosphate, triaryl phosphate, or mixtures thereof.

3. The functional fluid composition of clauses 1 and 2 wherein the one or more phosphate ester fluid base stocks comprise tri-n-butyl phosphate, tri-isobutyl phosphate, n-butyl di-isobutyl phosphate, di-isobutyl n-butyl phosphate, n-butyl diphenyl phosphate, isobutyl diphenyl phosphate, di-n-butyl phenyl phosphate, di-isobutyl phenyl phosphate, tri-n-pentyl phosphate, tri-isopentyl phosphate, triphenyl phosphate, isopropylated triphenyl phosphates, butylated triphenyl phosphates, or mixtures thereof.

4. The functional fluid composition of clauses 1-3 wherein the first erosion inhibitor comprises ammonium hexafluorophosphate (NH₄PF₆), N-benzyl-N,N,N-trimethyl ammonium hexafluorophosphate (CH₃)₃(CH₆H₅CH₂)NPF₆, potassium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, ammonium tetrafluoroborate (NH₄BF₄), sodium tetrafluoroborate, zinc tetrafluoroborate, sodium hexafluoroantimonate (NaSbF₆), ammonium hexafluoroantimonate, N-benzyl-N,N,N-triethyl phosphonium hexafluorophosphate, potassium hexafluoroantimonate, or mixtures thereof, and the second erosion inhibitor comprises potassium perfluoromethane sulfonate, potassium perfluoroethane sulfonate, sodium perfluorobutane sulfonate, sodium perfluorocyclohexane sulfonate, potassium perfluorooctane sulfonate, cesium perfluorooctadecane sulfonate, potassium perfluorocyclopentane sulfonate, potassium perfluoropentane sulfonate, or mixtures thereof.

5. The functional fluid composition of clauses 1-4 wherein the first erosion inhibitor comprises ammonium hexafluorophosphate (NH₄PF₆), N-benzyl-N,N,N-trimethyl ammonium hexafluorophosphate (CH₃)₃(CH₆H₅CH₂)NPF₆, potassium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, ammonium tetrafluoroborate (NH₄BF₄), sodium tetrafluoroborate, zinc tetrafluoroborate, sodium hexafluoroantimonate (NaSbF₆), ammonium hexafluoroantimonate, N-benzyl-N,N,N-triethyl phosphonium hexafluorophosphate, potassium hexafluoroantimonate, or mixtures thereof, and the second erosion inhibitor comprises potassium trifluoromethane sulfonate or potassium triflate; lithium trifluoromethane sulfonate or lithium triflate; potassium 1,1,2,2,2-pentafluoroethane-1-sulfonate; potassium 1,1,2,2,3,3,3-heptafluoropropane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,5-undecafluoropentane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,6-tridecafluorohexane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,7-pentadecafluoroheptane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctane-1-sulfonate; or mixtures thereof.

6. The functional fluid composition of clauses 1-5 wherein the one or more phosphate ester fluid base stocks comprise tri-n-butyl phosphate, and the erosion inhibitor mixture comprises potassium hexafluorophosphate and potassium trifluoromethane sulfonate.

7. The functional fluid composition of clauses 1-6 wherein the erosion inhibitor mixture is present in an amount sufficient to control one or more of electrical conductivity, acidity, copper corrosion inhibition, and hydrolytic stability, of the functional fluid composition.

8. The functional fluid composition of clauses 1-7 having an electrical conductivity of greater than 0.3 to less than 2.0 micromhos/cm determined in accordance with ASTM D2624, a total acid number from zero to less than 0.05 mg KOH/g determined in accordance with ASTM D974, a corrosiveness to copper of 1A, 1B and 2A determined in accordance with ASTM D130, and a water content from zero to less than 8000 ppm water determined in accordance with ASTM D6304.

9. The functional fluid composition of clauses 1-8 wherein the one or more phosphate ester fluid base stocks are present in an amount of at least 70 weight percent, based on the total weight of the functional fluid composition, and wherein the erosion inhibitor mixture is present in an amount from 0.01 weight percent to 0.15 weight percent, based on the total weight of the functional fluid composition.

10. A method for operating and lubricating a hydraulic system by utilizing as a hydraulic fluid a functional fluid comprising:

(a) a fluid base stock as a major component, wherein the fluid base stock comprises one or more phosphate ester fluid base stocks represented by the formula

wherein R¹, R² and R³ are independently a substituted or unsubstituted alkyl or aryl group; and

(b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises at least a first erosion inhibitor and a second erosion inhibitor; wherein the first erosion inhibitor comprises a perhalometallate or perhalometalloidate salt represented by the formula:

M[AX_(y)]_(z)

wherein A is a metal or metalloid; M is a solubilizing cation; X is a halogen; y is an integer from 1 to 7 and equal to the positive valence of A; and z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral; and wherein the second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid represented by the formula

R₁SO₃M

where M is an alkali metal, and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18; and wherein erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D1130 and ASTM D6304.

11. The method of clause 10 wherein the functional fluid has an electrical conductivity of greater than 0.3 to less than 2.0 micromhos/cm determined in accordance with ASTM D2624, a total acid number from zero to less than 0.05 mg KOH/g determined in accordance with ASTM D974, a corrosiveness to copper of 1A, 1B and 2A determined in accordance with ASTM D130, and a water content from zero to less than 8000 ppm water determined in accordance with ASTM D6304.

12. A method for identifying erosion inhibition effectiveness of a functional fluid, said method comprising:

providing a functional fluid comprising:

(a) a fluid base stock as a major component, wherein the fluid base stock comprises one or more phosphate ester fluid base stocks represented by the formula

wherein R¹, R² and R³ are independently a substituted or unsubstituted alkyl or aryl group; and

(b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises at least a first erosion inhibitor and a second erosion inhibitor; wherein the first erosion inhibitor comprises a perhalometallate or perhalometalloidate salt represented by the formula:

M[AX_(y)]_(z)

wherein A is a metal or metalloid; M is a solubilizing cation; X is a halogen; y is an integer from 1 to 7 and equal to the positive valence of A; and z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral; and wherein the second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid represented by the formula

R₁SO₃M

where M is an alkali metal, and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18;

determining electrical conductivity of the functional fluid in accordance with ASTM D2624;

determining corrosiveness to copper of the functional fluid in accordance with ASTM D130;

determining total acid number of the functional fluid in accordance with ASTM D974;

determining entrained water content in the functional fluid in accordance with ASTM D6304; and

identifying erosion inhibition effectiveness of the functional fluid based on results from one or more of ASTM D2624, ASTM D130, ASTM D974 and ASTM D6304.

13. The method of clause 12 wherein the functional fluid has an electrical conductivity of greater than 0.3 to less than 2.0 micromhos/cm determined in accordance with ASTM D2624, a total acid number from zero to less than 0.05 mg KOH/g determined in accordance with ASTM D974, a corrosiveness to copper of 1A, 1B and 2A determined in accordance with ASTM D130, and a water content from zero to less than 8000 ppm water determined in accordance with ASTM D6304.

14. A functional fluid composition comprising:

(a) a tri-n-butyl phosphate fluid base stock as a major component, and

(b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises potassium hexafluorophosphate and potassium trifluoromethane sulfonate; and wherein erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the potassium hexafluorophosphate or the potassium trifluoromethane sulfonate, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.

15. The functional fluid composition of clause 14 having an electrical conductivity of greater than 0.3 to less than 2.0 micromhos/cm determined in accordance with ASTM D2624, a total acid number from zero to less than 0.05 mg KOH/g determined in accordance with ASTM D974, a corrosiveness to copper of 1A, 1B and 2A determined in accordance with ASTM D130, and a water content from zero to less than 8000 ppm water determined in accordance with ASTM D6304.

All patents and patent applications, test procedures (such as ASTM methods, UL methods, and the like), and other documents cited herein are fully incorporated by reference to the extent such disclosure is not inconsistent with this disclosure and for all jurisdictions in which such incorporation is permitted.

When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. While the illustrative embodiments of the disclosure have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present disclosure, including all features which would be treated as equivalents thereof by those skilled in the art to which the disclosure pertains.

The present disclosure has been described above with reference to numerous embodiments and specific examples. Many variations will suggest themselves to those skilled in this art in light of the above detailed description. All such obvious variations are within the full intended scope of the appended claims. 

1. A functional fluid composition comprising: (a) a fluid base stock as a major component, wherein the fluid base stock comprises one or more phosphate ester fluid base stocks represented by the formula

wherein R¹, R² and R³ are independently a substituted or unsubstituted alkyl or aryl group; and (b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises at least a first erosion inhibitor and a second erosion inhibitor; wherein the first erosion inhibitor comprises a perhalometallate or perhalometalloidate salt represented by the formula: M[AX_(y)]_(z) wherein A is a metal or metalloid; M is a solubilizing cation; X is a halogen; y is an integer from 1 to 7 and equal to the positive valence of A; and z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral; and wherein the second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid represented by the formula R₁SO₃M where M is an alkali metal, and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18; and wherein erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.
 2. The functional fluid composition of claim 1 wherein the one or more phosphate ester fluid base stocks comprise trialkyl phosphate, dialkyl aryl phosphate, alkyl diaryl phosphate, triaryl phosphate, or mixtures thereof.
 3. The functional fluid composition of claim 1 wherein the one or more phosphate ester fluid base stocks comprise tri-n-butyl phosphate, tri-isobutyl phosphate, n-butyl di-isobutyl phosphate, di-isobutyl n-butyl phosphate, n-butyl diphenyl phosphate, isobutyl diphenyl phosphate, di-n-butyl phenyl phosphate, di-isobutyl phenyl phosphate, tri-n-pentyl phosphate, tri-isopentyl phosphate, triphenyl phosphate, isopropylated triphenyl phosphates, butylated triphenyl phosphates, or mixtures thereof.
 4. The functional fluid composition of claim 1 wherein, in the one or more phosphate ester fluid base stocks, R¹, R² and R³ are independently a substituted or unsubstituted alkyl group having from 4 to 9 carbon atoms, or an aryl group.
 5. The functional fluid composition of claim 1 wherein the first erosion inhibitor comprises ammonium hexafluorophosphate (NH₄PF₆), N-benzyl-N,N,N-trimethyl ammonium hexafluorophosphate (CH₃)₃(CH₆H₅CH₂)NPF₆, potassium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, ammonium tetrafluoroborate (NH₄BF₄), sodium tetrafluoroborate, zinc tetrafluoroborate, sodium hexafluoroantimonate (NaSbF₆), ammonium hexafluoroantimonate, N-benzyl-N,N,N-triethyl phosphonium hexafluorophosphate, potassium hexafluoroantimonate, or mixtures thereof; and the second erosion inhibitor comprises potassium perfluoromethane sulfonate, potassium perfluoroethane sulfonate, sodium perfluorobutane sulfonate, sodium perfluorocyclohexane sulfonate, potassium perfluorooctane sulfonate, cesium perfluorooctadecane sulfonate, potassium perfluorocyclopentane sulfonate, potassium perfluoropentane sulfonate, or mixtures thereof.
 6. The functional fluid composition of claim 1 wherein the first erosion inhibitor comprises ammonium hexafluorophosphate (NH₄PF₆), N-benzyl-N,N,N-trimethyl ammonium hexafluorophosphate (CH₃)₃(CH₆H₅CH₂)NPF₆, potassium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, ammonium tetrafluoroborate (NH₄BF₄), sodium tetrafluoroborate, zinc tetrafluoroborate, sodium hexafluoroantimonate (NaSbF₆), ammonium hexafluoroantimonate, N-benzyl-N,N,N-triethyl phosphonium hexafluorophosphate, potassium hexafluoroantimonate, or mixtures thereof; and the second erosion inhibitor comprises potassium trifluoromethane sulfonate or potassium triflate; lithium trifluoromethane sulfonate or lithium triflate; potassium 1,1,2,2,2-pentafluoroethane-1-sulfonate; potassium 1,1,2,2,3,3,3-heptafluoropropane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,5-undecafluoropentane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,6-tridecafluorohexane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,7-pentadecafluoroheptane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctane-1-sulfonate; or mixtures thereof.
 7. The functional fluid composition of claim 1 wherein the erosion inhibitor mixture comprises potassium hexafluorophosphate and potassium trifluoromethane sulfonate.
 8. The functional fluid composition of claim 1 wherein the one or more phosphate ester fluid base stocks comprise tri-n-butyl phosphate, and the erosion inhibitor mixture comprises potassium hexafluorophosphate and potassium trifluoromethane sulfonate.
 9. The functional fluid composition of claim 1 further comprising one or more of a viscosity index improver, antioxidant, acid scavenger, corrosion inhibitor, antifoam agent, and rust inhibitor.
 10. The functional fluid composition of claim 1 wherein the one or more phosphate ester fluid base stocks are present in an amount of at least 70 weight percent, based on the total weight of the functional fluid composition.
 11. The functional fluid composition of claim 1 wherein the erosion inhibitor mixture is present in an amount sufficient to control one or more of electrical conductivity, acidity, copper corrosion inhibition, and hydrolytic stability, of the functional fluid composition.
 12. The functional fluid composition of claim 1 having an electrical conductivity of greater than 0.3 to less than 2.0 micromhos/cm determined in accordance with ASTM D2624, a total acid number from zero to less than 0.05 mg KOH/g determined in accordance with ASTM D974, a corrosiveness to copper of 1A, 1B and 2A determined in accordance with ASTM D130, and a water content from zero to less than 8000 ppm water determined in accordance with ASTM D6304.
 13. The functional fluid composition of claim 1 wherein the erosion inhibitor mixture is present in an amount from 0.01 weight percent to 0.15 weight percent, based on the total weight of the functional fluid composition.
 14. The functional fluid composition of claim 1 wherein the first erosion inhibitor is present in an amount from 25 weight percent to 75 weight percent, and the second erosion inhibitor is present in an amount from 25 weight percent to 75 weight percent, based on the total weight of the erosion inhibitor mixture.
 15. The functional fluid composition of claim 1 wherein the weight ratio of the first erosion inhibitor to the second erosion inhibitor is from 25:75 to 75:25.
 16. The functional fluid composition of claim 1 wherein the functional fluid comprises an aircraft hydraulic fluid.
 17. A method for operating and lubricating a hydraulic system by utilizing as a hydraulic fluid a functional fluid comprising: (a) a fluid base stock as a major component, wherein the fluid base stock comprises one or more phosphate ester fluid base stocks represented by the formula

wherein R¹, R² and R³ are independently a substituted or unsubstituted alkyl or aryl group; and (b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises at least a first erosion inhibitor and a second erosion inhibitor; wherein the first erosion inhibitor comprises a perhalometallate or perhalometalloidate salt represented by the formula: M[AX_(y)]_(z) wherein A is a metal or metalloid; M is a solubilizing cation; X is a halogen; y is an integer from 1 to 7 and equal to the positive valence of A; and z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral; and wherein the second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid represented by the formula R₁SO₃M where M is an alkali metal, and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18; and wherein erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.
 18. The method of claim 17 wherein the one or more phosphate ester fluid base stocks comprise trialkyl phosphate, dialkyl aryl phosphate, alkyl diaryl phosphate, triaryl phosphate, or mixtures thereof.
 19. The method of claim 17 wherein the one or more phosphate ester fluid base stocks comprise tri-n-butyl phosphate, tri-isobutyl phosphate, n-butyl di-isobutyl phosphate, di-isobutyl n-butyl phosphate, n-butyl diphenyl phosphate, isobutyl diphenyl phosphate, di-n-butyl phenyl phosphate, di-isobutyl phenyl phosphate, tri-n-pentyl phosphate, tri-isopentyl phosphate, triphenyl phosphate, isopropylated triphenyl phosphates, butylated triphenyl phosphates, or mixtures thereof.
 20. The method of claim 17 wherein, in the one or more phosphate ester fluid base stocks, R¹, R² and R³ are independently a substituted or unsubstituted alkyl group having from 4 to 9 carbon atoms, or an aryl group.
 21. The method of claim 17 wherein the first erosion inhibitor comprises ammonium hexafluorophosphate (NH₄PF₆), N-benzyl-N,N,N-trimethyl ammonium hexafluorophosphate (CH₃)₃(CH₆H₅CH₂)NPF₆, potassium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, ammonium tetrafluoroborate (NH₄BF₄), sodium tetrafluoroborate, zinc tetrafluoroborate, sodium hexafluoroantimonate (NaSbF₆), ammonium hexafluoroantimonate, N-benzyl-N,N,N-triethyl phosphonium hexafluorophosphate, potassium hexafluoroantimonate, or mixtures thereof; and the second erosion inhibitor comprises potassium perfluoromethane sulfonate, potassium perfluoroethane sulfonate, sodium perfluorobutane sulfonate, sodium perfluorocyclohexane sulfonate, potassium perfluorooctane sulfonate, cesium perfluorooctadecane sulfonate, potassium perfluorocyclopentane sulfonate, potassium perfluoropentane sulfonate, or mixtures thereof.
 22. The method of claim 17 wherein the first erosion inhibitor comprises ammonium hexafluorophosphate (NH₄PF₆), N-benzyl-N,N,N-trimethyl ammonium hexafluorophosphate (CH₃)₃(CH₆H₅CH₂)NPF₆, potassium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, ammonium tetrafluoroborate (NH₄BF₄), sodium tetrafluoroborate, zinc tetrafluoroborate, sodium hexafluoroantimonate (NaSbF₆), ammonium hexafluoroantimonate, N-benzyl-N,N,N-triethyl phosphonium hexafluorophosphate, potassium hexafluoroantimonate, or mixtures thereof; and the second erosion inhibitor comprises potassium trifluoromethane sulfonate or potassium triflate; lithium trifluoromethane sulfonate or lithium triflate; potassium 1,1,2,2,2-pentafluoroethane-1-sulfonate; potassium 1,1,2,2,3,3,3-heptafluoropropane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,5-undecafluoropentane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,6-tridecafluorohexane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,7-pentadecafluoroheptane-1-sulfonate; potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctane-1-sulfonate; or mixtures thereof.
 23. The method of claim 17 wherein the erosion inhibitor mixture comprises potassium hexafluorophosphate and potassium trifluoromethane sulfonate.
 24. The method of claim 17 wherein the one or more phosphate ester fluid base stocks comprise tri-n-butyl phosphate, and the erosion inhibitor mixture comprises potassium hexafluorophosphate and potassium trifluoromethane sulfonate.
 25. The method of claim 17 wherein the functional fluid further comprises one or more of a viscosity index improver, antioxidant, acid scavenger, corrosion inhibitor, antifoam agent, and rust inhibitor.
 26. The method of claim 17 wherein the one or more phosphate ester fluid base stocks are present in an amount of at least 70 weight percent, based on the total weight of the functional fluid composition.
 27. The method of claim 17 wherein the erosion inhibitor mixture is present in an amount sufficient to control one or more of electrical conductivity, acidity, copper corrosion inhibition, and hydrolytic stability, of the functional fluid composition.
 28. The method of claim 17 wherein the functional fluid has an electrical conductivity of greater than 0.3 to less than 2.0 micromhos/cm determined in accordance with ASTM D2624, a total acid number from zero to less than 0.05 mg KOH/g determined in accordance with ASTM D974, a corrosiveness to copper of 1A, 1B and 2A determined in accordance with ASTM D130, and a water content from zero to less than 8000 ppm water determined in accordance with ASTM D6304.
 29. The method of claim 17 wherein the erosion inhibitor mixture is present in an amount from 0.01 weight percent to 0.15 weight percent, based on the total weight of the functional fluid composition.
 30. The method of claim 17 wherein the first erosion inhibitor is present in an amount from 25 weight percent to 75 weight percent, and the second erosion inhibitor is present in an amount from 25 weight percent to 75 weight percent, based on the total weight of the erosion inhibitor mixture.
 31. The method of claim 17 wherein the weight ratio of the first erosion inhibitor to the second erosion inhibitor is from 25:75 to 75:25.
 32. The method of claim 17 wherein the functional fluid comprises an aircraft hydraulic fluid.
 33. A method for identifying erosion inhibition effectiveness of a functional fluid, said method comprising: providing a functional fluid comprising: (a) a fluid base stock as a major component, wherein the fluid base stock comprises one or more phosphate ester fluid base stocks represented by the formula

wherein R¹, R² and R³ are independently a substituted or unsubstituted alkyl or aryl group; and (b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises at least a first erosion inhibitor and a second erosion inhibitor; wherein the first erosion inhibitor comprises a perhalometallate or perhalometalloidate salt represented by the formula: M[AX_(y)]_(z) wherein A is a metal or metalloid; M is a solubilizing cation; X is a halogen; y is an integer from 1 to 7 and equal to the positive valence of A; and z is an integer from 1 to 3 and sufficient to maintain the salt electro-neutral; and wherein the second erosion inhibitor comprises an alkali metal salt of perfluoroalkyl sulfonic acid represented by the formula R₁SO₃M where M is an alkali metal, and R₁ is a C_(n)F_(2n+1) or a cyclic C_(a)F_(2a−1) group where n is an integer of from 1 to 18 and a is an integer from 4 to 18; determining electrical conductivity of the functional fluid in accordance with ASTM D2624; determining corrosiveness to copper of the functional fluid in accordance with ASTM D130; determining total acid number of the functional fluid in accordance with ASTM D974; determining entrained water content in the functional fluid in accordance with ASTM D6304; and identifying erosion inhibition effectiveness of the functional fluid based on results from one or more of ASTM D2624, ASTM D130, ASTM D974 and ASTM D6304.
 34. The method of claim 33 wherein the functional fluid has an electrical conductivity of greater than 0.3 to less than 2.0 micromhos/cm determined in accordance with ASTM D2624, a total acid number from zero to less than 0.05 mg KOH/g determined in accordance with ASTM D974, a corrosiveness to copper of 1A, 1B and 2A determined in accordance with ASTM D130, and a water content from zero to less than 8000 ppm water determined in accordance with ASTM D6304.
 35. A functional fluid composition comprising: (a) a tri-n-butyl phosphate fluid base stock as a major component, and (b) an erosion inhibitor mixture as a minor component; wherein the erosion inhibitor mixture comprises potassium hexafluorophosphate and potassium trifluoromethane sulfonate; and wherein erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the potassium hexafluorophosphate or the potassium trifluoromethane sulfonate, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304.
 36. The functional fluid composition of claim 35 wherein the weight ratio of the potassium hexafluorophosphate to the potassium trifluoromethanesulfonate is 30:70 or 40:60.
 37. The functional fluid composition of claim 35 wherein the erosion inhibitor mixture is present in an amount sufficient to control one or more of electrical conductivity, acidity, copper corrosion inhibition, and hydrolytic stability, of the functional fluid composition.
 38. The functional fluid composition of claim 35 having an electrical conductivity of greater than 0.3 to less than 2.0 micromhos/cm determined in accordance with ASTM D2624, a total acid number from zero to less than 0.05 mg KOH/g determined in accordance with ASTM D974, a corrosiveness to copper of 1A, 1B and 2A determined in accordance with ASTM D130, and a water content from zero to less than 8000 ppm water determined in accordance with ASTM D6304. 